First, a note on the support template: it exists for a reason. You have not filled in any of the requested fields — PHP version, web server, database, config.php excerpt, Nextcloud log entries, nothing. Without that information, any answer is guesswork, which is likely why this thread has had no replies so far. Please fill in the template for future posts — it makes a significant difference in the quality and speed of help you receive.
That said, since you mentioned AIO 12.9.2, here is what can be said.
Your check in Adobe Acrobat Reader confirms the PDFs themselves are valid — but that has no relation to Nextcloud’s thumbnail generation. Nextcloud does not use Acrobat; it generates previews independently on the server side.
This is how PDF previews work:
PDF thumbnails are generated either by Collabora (if enabled — which it is in most AIO setups) or by the imaginaryPDF preview provider. The results are cached in data/appdata_<instanceid>/preview/. Nextcloud never reads the PDF on the client side for this purpose.
The most common causes for selective preview failures:
- Encrypted or password-protected PDFs — the preview generator cannot open them
- PDFs with complex content (certain fonts, embedded streams, newer spec features) that the generator does not fully support
- Memory or timeout limits — very large or complex PDFs may exceed the limits configured for the preview service
- A previously failed preview is cached — once generation fails, Nextcloud may not retry automatically
- Check whether the affected PDFs are encrypted or have any access restrictions.
- Run
occ preview:generate-allto force a retry for all files. - Check nextcloud.log filtered for
previewaround the time you upload one of the affected files — it may say exactly why generation failed.
But, without knowing your exact preview provider configuration — which provider is active, and what the log says — the above can only narrow down the likely causes. If none of these points resolve it, please share your config.php (sensitive values redacted) and the relevant log entries, so a more targeted diagnosis is possible.
h.t.h.
ernolf